1. Field of the Invention
The invention relates to apparatus and methods for delivery of neutron beams for medical therapy. More particularly, the invention relates to a small neutron generator using a high current electron bombardment ion source and methods of treating tumors therewith.
2. State of the Art
Application of neutrons for radiotherapy of cancer has been a subject of considerable clinical and research interest since the discovery of the neutron by Chadwick, in 1932. Fast neutron radiotherapy was first used by Robert Stone in the Lawrence Berkeley Laboratory in 1938.
This technology has evolved over the years to the point where it is now a reimbursable modality of choice for inoperable salivary gland tumors, and it is emerging, on the basis of recent research data, as a promising alternate modality for prostate cancer, some lung tumors, and certain other malignancies as well.
Neutron generators presently used in neutron therapy comprise either a particle accelerator (tandem or proton linear cyclotron), which bombards a beryllium target with its particles (protons or deuterium nuclei called deuterons) of energy between 15 and 60 MeV, or a particle accelerator, which bombards a tritiated target of deuterons of 75 to 500 KeV, or which bombards a hydrogenatable metal target (occluded “autotarget”, this target being regeneratable) with a mixture of deuterons and tritium nuclei (called tritons) of 75 to 500 KeV, so as to produce neutrons of energy equal to 14 MeV, which are very effective in neutron therapy. The process is referred to as the DT reaction. Also, neutrons of energy equal to 2.5 MeV are produced when 75-500 KeV deuterons strike deuterium atoms in the target. The process is referred to as the DD Reaction.
A typical prior art neutron generator for neutron therapy uses a plasma discharge source, Penning ionization gauge, capable of developing milli-amperes of ion current. High voltage is typically 100 KeV, resulting in target power dissipation on the order of 100 watts. Dose at 1 meter from the target can be 100 rem/hr which requires considerable radiation protection measures for operation in a laboratory or medical treatment facility.
Considerable work has also been carried out for development of thermonuclear plasma type neutron sources. These devices have relatively large chambers, 10's of cm in radius, to contain the reactant gas, and require relatively large power sources per neutron produced, because the relative energy difference of the particles is low compared with 120 keV which is the peak of the cross section for the DT reaction.
Prior art neutron therapy systems are largely located only at major research centers since they are physically complex, bulky, and require high-level operating staffs to maintain. In general these systems are not well suited for wide-spread, practical, clinical deployment. Moreover, due to their substantial power requirements, none of these systems are suitable for field use.
Recently, there have been advances in brachytherapy, i.e. radiation therapy where a neutron source is placed in contact with the tumor. The procedures most frequently used involve the implantation of radioactive “seeds” which are delivered to the treatment site with hollow needles. One of the most promising neutron sources for brachytherapy is Californium-252. Californium-252 sources are unique in providing a high intensity source of neutrons in a compact and portable package. The operational and safety requirements of Cf-252 sources are onerous.
Clinical research with Cf-252 neutron brachytherapy has been hampered by radiation safety difficulties, including source handling, source transport, staff and area monitoring and shielding. The complex regulatory and shielding requirements alone are enough to discourage university hospitals and clinics from implementing Cf-252 brachytherapy and participating in this important area of clinical research. Theoretical understanding of the research is complicated by the fact that Cf-252 neutrons are produced in a broad energy spectrum, with 40% of the dose from fission gamma rays. If neutron brachytherapy is dramatically successful, it is not clear whether the world supply of Cf-252 (produced in high flux reactors) will be capable of meeting the demand from thousands of treatment centers throughout the world.
Although radioactive seed therapy may be a significant improvement over therapy which uses large neutron generators, it does have drawbacks. In addition to the issues discussed above, it is still a surgical procedure which requires high skill and a controlled environment. Implanting and subsequently removing the seeds is a very meticulous task.